The present invention relates to detecting apparatus, such as a radar system, a scanning sonar or a fish-finding echo sounder, which detect targets or other objects by transmitting propagating waves like radio waves or sound waves in specified directions and receiving reflected waves from those objects.
In radar systems installed on vessels, for example, there exist conventional techniques which enable an operator to discriminate the levels of received signals from an on-screen image. Specifically, a monochrome radar controls its on-screen display in such a manner that echoes are displayed at different levels of brightness which are approximately proportional to the levels of the received signals, whereas a color radar controls its on-screen display in such a manner that echoes are displayed in different colors depending on the levels of the received signals.
Generally, the higher the target strength, the higher the levels of the received signals. Thus, in the aforementioned monochrome radar, for example, objects having high target strengths are displayed with high brilliance while objects having low target strengths are displayed with lower brilliance. In the aforementioned color radar, on the other hand, objects having high target strengths are painted in red while objects having low target strengths are displayed in green, for example.
These conventional radar systems are associated with a problem that it is difficult to distinguish true echoes reflected by vessels or ground, for instance, when the received echo signals contain unwanted signals, such as reflections from the sea surface, rain or snow. Although it is possible to suppress these unwanted signals to a certain degree by means of a sensitivity time control (STC) circuit and a fast time constant (FTC) circuit, these circuits do not always exhibit optimum response characteristics but could adversely affect true target echoes depending on their settings.
Although the conventional monochrome radar displays short-range targets with high brilliance and long-range targets with low brilliance on average. But there are cases small targets are shown with low brilliance in a short-range or large targets are shown with low brilliance in a long-range. It is therefore difficult to distinguish the types of targets simply from their brilliance on a screen. This is true in the conventional color radar as well.
The above fact also applies to detecting apparatus like scanning sonars and fish-finding echo sounders which utilize sound waves. More specifically, since both fish close to the bottom and the bottom itself similarly return strong echoes of sound waves, it is occasionally difficult to discriminate fish schools from the brilliance or display colors of their echoes on these detecting apparatus.
It is an object of this invention to solve the aforementioned problems and, more particularly, to provide detecting apparatus which enable easy discrimination of target types, or easy discrimination of desired and undesired targets.
In one principal form of the invention, a detecting apparatus detects targets by periodically transmitting propagating waves in a specific detecting direction and receiving echoes reflected by the targets, wherein display brilliance or color is varied between a blip exhibiting large fluctuation in received signal strength and a blip exhibiting small fluctuation in received signal strength from one transmit/receive cycle to another, the blips being detected at the same point or at nearby points in the detecting direction.
In a radar system which repeatedly scans in successively varying antenna directions, for example, a target like a land or an anchored vessel produces practically the same received signal strength from one scanning cycle to another when observed within a short time span. In contrast, the received signal strength of echoes due to reflections from rain or snow or sea clutter varies within a short period of time, e.g., from one scanning cycle to another. According to the invention, display brilliance or color is varied between a blip exhibiting large fluctuation in received signal strength and a blip exhibiting small fluctuation in received signal strength from one scanning cycle to another. This makes it easy to distinguish target types. Specifically, it becomes easy tell whether a particular target is a stable one like land or a vessel, an unstable one like a navigational buoy or a bird whose received signal strength fluctuates within a relatively short period of time due to their swing motion, movement, or such a target as rain, snow or sea clutter whose received signal strength is low fluctuates within an extremely short period of time based on their display brilliance or color.
In a scanning sonar or a fish-finding echo sounder, practically the same received signal strength is obtained from sea bottom reflection from one transmit/receive cycle to another when observed within a short time span. However, a moving fish school, for instance, produces a received signal strength that varies within a short period of time, e.g., every transmit/receive cycle. Since the display brilliance or color is varied between a blip exhibiting large fluctuation in received signal strength and a blip exhibiting small fluctuation in received signal strength from one transmit/receive cycle to another in the present invention, it is possible to easily recognize whether a particular target is a stable one like the seabed or an unstable one like a fish school whose received signal strength fluctuates within a short period of time based on their display brilliance or color.
In the context of this Specification, the expression xe2x80x9ctargetxe2x80x9d is used to mean not only a single target that is searched for for specific purposes but also any objects which reflect such propagating waves as electromagnetic waves or ultrasonic waves, including a continuous mass of a object, scattered objects, or a group of scattered objects. Thus, a vessel, land, buoy, rain, snow and sea surface are all xe2x80x9ctargetsxe2x80x9d for the radar system. Likewise, fish schools, seabed and large-sized individual fish are all xe2x80x9ctargetsxe2x80x9d for the scanning sonar and fish-finding echo sounder.
In another principal form of the invention, a detecting apparatus detects targets by periodically transmitting propagating waves in a specific detecting direction and receiving echoes reflected by the targets, wherein the pattern of fluctuation in display brilliance or color is varied according to fluctuation in received signal strength at the same point or at nearby points in the detecting direction from one transmit/receive cycle to another depending on whether the received signal strength has once exceeded a specified value or not.
In a radar system, for example, a navigational buoy or a seabird has a high target strength against electromagnetic waves so that high received signal strengths are obtained at specific frequency although the received signal is not necessarily strong in every scannin cycle. Even with this kind of target whose received signal strength is unstable, its on-screen echo is presented with a different pattern of fluctuation in display brilliance or color compared to reflections from other targets such as rain, snow and sea surface, once that target is received with a high received signal strength. This makes it easy to distinguish target types.
In one aspect of the invention, the detecting apparatus comprises a signal processor which determines a new output value corresponding to new display brilliance or color based on input values including a previous output value representative of previous display brilliance or color and a value representative of a new received signal strength, wherein the signal processor makes such determination of the new output value by reference to a table containing the received signal strength and the previous output value as header words and candidates for the new output value as data contents.
This structure enables a wide variety of mathematical formulae to be implemented for determining the new output value based on the previous output value and the new received signal strength, thereby allowing diversification of relationship between fluctuation in received signal strength and variation in display brilliance or color. In addition, this structure enables high-speed determination of the new output value.
In another aspect of the invention, the signal processor determines the new output value by using a mathematical equation of which input variables are the received signal strength and the previous output value and output variable is the new output value.
This structure makes it possible to perform the same function as achieved with the aforementioned table while eliminating the need for memory capacity for accommodating the table.
In still another aspect of the invention, the radar apparatus comprises a transmitter which substantially periodically transmits search signals in a direction, a receiver which receives echoes reflected by targets, and a signal processor which varies display brilliance or color depending on the degree of signal strength fluctuations of received echo signals resulting from a plurality of search signals and produced at the same points or nearby points in said direction.